Two major categories of phenolic resins are known. These are the novolak resins and the phenolic resole resins. Novolak resins are prepared by the reaction of excess phenol with formaldehyde under strongly acidic conditions where the formaldehyde to phenol ratio is typically from 0.5:1.0 to 1.0:1.0. Novolak resins are linear or slightly branched condensation products linked together by methylene bridges, and have relatively low molecular weight, i.e. up to approximately 2,000. They are soluble and permanently fusible, i.e. thermoplastic. Novolaks are cured with hexamethylenetetramine (HEXA). In foundry applications, novolak resins are almost exclusively used in the "shell process" where foundry sand is precoated with the resin before curing with HEXA.
On the other hand, phenolic resole resins are prepared by the reaction of phenol and excess formaldehyde under alkaline conditions. Formaldehyde to phenol ratios between 1.1:1.0 to 3.0:1.0 are customarily used. Phenolic resole resins are somewhat stable at room temperature, but are transformed into three dimensional, crosslinked, insoluble, and infusible polymers by the application of heat.
It is generally accepted that the best overall performance characteristics for phenolic-urethane foundry binders are achieved by using highly ortho substituted phenolic resole resins having benzylic ether and methylene bridging units between the constituent aromatic rings where the ratio of ether to methylene bridging units is at least 1.0, i.e. benzylic ether phenolic resole resins. These benzylic ether phenolic resole resins are produced by the reaction of phenol with formaldehyde in the presence of a divalent metal catalyst at atmospheric pressure with reflux such that the formaldehyde to phenol ratio in the reaction charge is generally from 1.0:1.0 to 2.0:1.0.
The preparation of benzylic ether phenolic resole resins involves the reaction of phenol and formaldehyde in the presence of a divalent metal catalyst in an open reaction vessel at increased temperature and the continuous stripping of water formed by the reaction from the reaction mixture. See U.S. Pat. Nos. 3,409,579; 3,485,797; and 4,546,124.
It is known to use pressure in the preparation of novolak resins. R. W. Martin in "The Chemistry of Phenolic resole resins," John Wiley & Sons, 1956, p. 111 mentions preparation of novolak resins without catalysts at high temperatures in autoclaves. A. Knop and L. A. Pilato in "Phenolic resole resins Chemistry, Applications and Performance," Springer-Verlag, 1985, pp. 93-95 mentions the continuous production of novolak resins which involves heating at 120.degree. C.-180.degree. C. at up to 100 psig to enhance the reaction rate. A process for preparing novolak resins under pressure is disclosed in U.S. Pat. No. 3,687,896, although it is not clear how the pressure in the reaction vessel is obtained. These novolak resins prepared under pressure are not highly ortho substituted and contain only methylene bridges between the aromatic rings.